Biaxially stretched polypropylene films have superior electrical characteristics such as high withstand voltage performance and low dielectric loss, and due to their high moisture resistance, are widely used as dielectric films for capacitors.
Polypropylene films for capacitors are preferably used in high-voltage capacitors as well as various types of switching power supplies, converters, inverters such as filters, and capacitors used for smoothing. In recent years, the demand for reduced size and higher capacitance of capacitors has become extremely strong, and the demand for reduced thickness of films for capacitors is also increasing.
Moreover, polypropylene film capacitors are beginning to be widely used as smoothing capacitors in inverter power supply circuits in order to control drive motors used in electric vehicles, hybrid vehicles and the like for which demand has increased in recent years.
The capacitors for the inverter power supply equipment used in such vehicles and the like are required to be compact, lightweight and have high capacitance while continuing to operate stably and withstand high direct current voltage over a long period of time and over a wide temperature range of −40° C. to 90° C., or in other words, are required to continue to maintain their electrostatic capacitance.
Consequently, dielectric films for capacitors used in the aforementioned capacitors for inverter power supply equipment are required to have high stretching performance that enables them to be stretched to an extremely thin thickness of 1 μm to 6 μm, and have high withstand voltage characteristics such that the film does not undergo breakdown even if subjected to higher direct current voltages at higher temperatures, or in other words, are required to have improved breakdown voltage. In addition, in capacitors composed of such a film, improvement of long-term durability such that the film does not break down even if continuously subjected to high temperatures and high voltages as described above for a long period of time, namely minimizing time-based changes in electrostatic capacitance, is essential.
A long known method for improving withstand voltage characteristics consist of improving the breakdown voltage value of a film by controlling the crystallinity of a polypropylene resin and the smoothing performance of the film surface. For example, Patent Document 1 discloses a capacitor composed of a highly stereoregular polypropylene resin containing an antioxidant. In addition, Patent Document 2, for example, discloses a technology relating to a film that realizes a high melt crystallization temperature (high crystallinity) and control of surface smoothing performance of the film, and a capacitor thereof, by using a polypropylene resin having high melt tension. However, since simply increasing the stereoregularity or crystallinity of a polypropylene resin leads to a decrease in film stretchability causing the film to break easily in the stretching process, this is not preferable in terms of production. In addition, the technology described in Patent Document 2 is unable to adequately respond to the demands of the rapidly developing capacitor market.
In order to obtain an ultrathin film as described above, it is essential to improve the stretchability of the resin and cast sheet. However, improving this characteristic, namely stretchability, is typically in conflicting with techniques for improving withstand voltage property, or in other words, increasing crystallinity of a polypropylene resin, as previously described.
In contrast, Patent Document 3 discloses a finely surface-roughened film obtained by stretching from a cast stock having a comparatively low level of β crystals using a polypropylene resin having molecular weight distribution and stereoregularity within specific ranges. Since the finely surface-roughened film stretched from a cast stock having a comparatively low level of β crystals is a thin film having withstand voltage characteristics and suitable surface roughness, it is a finely surface-roughened film that attains a level capable of satisfying the aforementioned three characteristics, namely stretching performance, withstand voltage characteristics and long-term durability. However, there is still room for improvement in order to satisfy severe required standards relating to withstand voltage property at high temperatures.
Moreover, Patent Document 4 discloses the realization of both high withstand voltage performance and reduced film thickness without increasing the stereoregularity of a polypropylene resin as a result of adjusting molecular weight distribution by containing a low molecular weight component in the polypropylene resin. However, this is unable to attain a level able to adequately satisfy the withstand voltage performance required by the market.
On the other hand, Patent Document 5 discloses a technology for adjusting Z-average molecular weight (Mz) to realize abroad molecular weight distribution, namely that in which Mw/Mn≧5.4 and Mz/Mn≧20, as an example of a technology for adjusting high molecular weight components of a polypropylene resin. Here, Mw refers to weight average molecular weight and Mn refers to number average molecular weight. However, although effects relating to β crystal formation and film moldability are improved as a result of realizing a molecular weight distribution having such a range, adequate studies have not been carried out with respect to high heat resistance and high withstand voltage, and there can still be said to be room for improvement.
Moreover, a technology for obtaining a polypropylene resin having a broad molecular weight distribution by adjusting high molecular weight components using a single polymerization step, namely a simple method, through the use of catalyst technology, and a technology relating to a film that uses that resin, have been disclosed (Patent Document 6). However, even according to these technologies, it is still considered to be difficult to adequately respond to market needs by realizing both an extremely thin film thickness and withstand voltage property.
In addition, as is disclosed in Patent Document 1 as well, an antioxidant is known to have at least some effect on long-term withstand voltage performance and capacitor electrical performance.
Patent Document 7 discloses a technology for suppressing the dielectric loss of a film to a low level by using a suitable combination and incorporated amounts of phenol-based antioxidants. However, there is nothing exemplified or suggested regarding the service life of a capacitor when subjected to high voltage, or in other words, long-term durability and withstand voltage property at high temperatures. In addition, as a more recent technology, Patent Document 8 discloses a technology for improving insulation resistance of a film at high temperatures by using an antioxidant having a high melting point. However, there is nothing exemplified or suggested in the technology of Patent Document 8 relating to withstand voltage property at high temperatures when subjected to high voltage.
In this manner, the technologies described in Patent Documents 1 to 8 are still unable to satisfy the rigorous demands of the rapidly developing capacitor industry, namely rigorous demands relating to withstand voltage performance of films when loaded at higher voltages at high temperatures.